…starting with Firefox 42 but you need to go to the Mozilla FTP site in order to download it. Once you click the link, find the most current version of Firefox and click that link, then click on the win64 link. Choose your localized (language) build then download the .exe setup file. At sometime in the future, the Win64 build will be offered directly from the Firefox downloads page.
Per Bug 1181014, 32-bit Windows users who are running 64-Bit version of Windows will be offered an updated to the 64-bit version. This likely won’t happen until sometime next year after some more testing is done to insure this won’t create new problems.